Cube maps aren’t supported yet in the shader generator. It’s just unfinished code.
As for the performance hit: no, it’s not rendering the scene twice. But it is using shaders and per-pixel lighting on the whole scene. That is a performance hit. In general, I think it’s an acceptable performance hit: most games use per-pixel lighting these days.
It’s also doing an image postprocessing filter: that’s a performance hit too.